The present invention relates to improvements in and of the method for preparing concrete by use of a multi-layer pan type mixer as disclosed by the present inventors in Japanese Patent Publication No. Sho 53-31167 (hereinafter called the earlier invention).
According to the earlier invention, a multi-layer pan type mixer is used, which comprises upper and lower mixing tanks having their main shafts connected to each other directly or through an intermediate transmission device, a single motor for driving both the upper and lower mixing tanks, and a discharge gate communicating between the upper tank and the lower mixing tanks. Sand, cement and water are successively fed into the upper mixing tank to perform primary mixing to obtain a mortar, and the mortar thus obtained is discharged into the lower mixing tank. At the same time, gravel is fed into the lower mixing tank to be dispersed in the mortar, whereafter secondary mixing is performed.
According to this earlier invention, remarkable advantages are obtained in that high-quality concrete can be prepared and the efficiency of the mixer can be improved. On the other hand, since all of the mixing water is supplied to the mixing tank of the upper mixer, and no secondary mixing water is supplied to the mixing tank of the lower mixer, this prior art has the disadvantages mentioned below.
In the practical preparation of concrete, the mixing proportions of the materials and the amount of mixing water, are adjusted so as to obtain ready-mixed concrete of various types as required for various kinds of secondary concrete products. In the earlier invention, when a concrete of the desired properties is made by adjusting the amount of mixing water and other materials, all of the mixing water is fed into the mixing tank of the upper mixer. Therefore, in a case where the amount of mixing water is large, the friction among the mortar materials becomes so small that satisfactory mixing cannot be achieved. Therefore, in the earlier invention, it is necessary to use a mixing adjusting means for increasing the rate of rotation of the mixer or to modify the shape of the mixing blades so as to enhance the mixing efficiency.